Creating models (levels) for a Unity game. Do I create 1 model or several?
Creating smaller parts (i.e. separate rooms, halls, corners) is referred to as "Modular structuring".

Modularity is faster, easier to get great results from, and is easier to change up and reuse, but it comes at the cost of adaptability, so this really depends on your game. If you're doing a level where everything is uniform anyway, like a subway tunnel or the interior of a military base or vehicle, modularity can be your best friend. Other cases, like a ranch house or an art center (cause they usually involve weird architecture) can create serious problems for the modular output due to the fact that you would be creating so many different components you might as well just do the whole thing by hand. It's really a case by case study, but I generally use a mix, modular where it works, and conventional where it doesn't.

If you do chose to use some modular structuring, you should follow a few rules:


Always use standardized sizes (this will help with assembly, avoiding trying to find the happy-medium between a 2.5 foot door and a 3 foot door.)
make everything the same theme
Place the origin of the modular sections at one edge or corner. (so you can use snapping to position them.)


someone has a great set of tutorials on modular structuring, but I can't find it right now, so I'll post a link when I do.